The gastrointestinal (GI) microbiota plays a critical role in determining the immunologic outcome of various signaling events in host cells via their gene products, exceeding the human genome by a hundredfold (Ley et al., 2006; Qin et al., 2010). As such, the composition of the GI microbiota and host immunity are mutualistic and continuously influence each other (Maslowski and Mackay, 2011; McDermott and Huffnagle, 2014).
Intestinal homeostasis is tightly controlled by regulatory immune mechanisms, which are established by the interactions of the trillions of microbes and their gene products with numerous pattern recognition receptors (PRRs), including C-type lectin receptors (CLRs), such as SIGNR3 (Konstantinov et al., 2008; Osorio and Reis e Sousa, 2011). Disruption of this delicate balance by inimical signals has devastating consequences that may result in intestinal disorders, including inflammatory bowel disease (IBD). When this occurs, highly activated innate cells trigger intestine-infiltrating pathogenic T cell subsets (e.g., Th1, Th17), and regulatory T cells (Tregs) with pro-inflammatory characteristics (Geremia et al., 2014; Khazaie et al., 2012; Neurath, 2014) that ultimately drive tissue destruction and intestinal disease progression. Innate cells (e.g., dendritic cells, macrophages) are the initial targets of either culpable microbes or their gene products, which subsequently affect the regulation/stimulation of intestinal immunity (Atarashi et al., 2013; Ivanov and Honda, 2012; Yang et al., 2014). Given these entwined relationships, it is not surprising that microbial products have been linked to the pathology of intestinal auto-inflammation (Nicholson et al., 2012). The underlying associations between gut microbes and inflammatory diseases (e.g., IBD) have been well documented; however, the cellular and molecular mechanisms by which intestinal commensal gene product(s) and their molecular receptor(s) impact immune responses remain unclear.
Information regarding the immunobiologic functions of Lactobacillus acidophilus surface layer proteins (Slps) is relatively limited. Slps are paracrystalline (glyco) protein arrays that are abundantly present on the cell surfaces of most eubacteria and archaea, including L. acidophilus (Johnson et al., 2013). L. acidophilus NCFM possesses three Slp-encoding genes: slpA (LBA0169), slpB (LBA0175), and slpX (LBA0512) (Goh et al., 2009). Diverse functional roles have been proposed for Slps, including cell shape determinants, molecular sieves, protective layers against viral infection, anchoring sites for surface-associated enzymes and facilitators of cellular adhesion through PRRs, including C-type lectins (CLECs) (Konstantinov et al., 2008).
CLECs recognize carbohydrate structures on self and non-self antigens (Engering et al., 2002; Osorio and Reis e Sousa, 2011). Eighteen CLECs, including DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), have been identified on dendritic cells (DCs) and macrophages (MΦs) (Ehlers, 2010; van Kooyk and Geijtenbeek, 2003). DC-SIGN, which was previously shown to bind L. acidophilus-SlpA in vitro (Konstantinov et al., 2008), is a calcium-dependent carbohydrate-binding protein with specificity for the mannose-containing glycans of microbial surface components and fucose-containing Lewis antigens (Ehlers, 2010). Of the eight murine homologs of DC-SIGN, SIGNR3 (CD209d) exhibits the most biochemical similarity to human DC-SIGN (Powlesland et al., 2006).
SIGNR3 contains a carbohydrate recognition domain (CRD) and signals through a hemi-immunoreceptor tyrosine-based activation motif (hemi-ITAM) pathway (Tanne et al., 2009). Such signaling potentially downregulates the ubiquitously expressed leukotriene A4 hydrolase (LTA4H) (Tobin et al., 2010) that catalyzes proinflammatory leukotriene B4 (LTB4) synthesis from LTA4 (Snelgrove et al., 2010), which consequently activates interleukin (IL)-1β production. Here, we identify SlpA as a key effector molecule expressed by L. acidophilus, and demonstrate its in vivo protective role in murine colitis models. Moreover, we provide evidence that protection by L. acidophilus-SlpA is conferred via signaling through a single CLR, namely SIGNR3.
As discussed above, normal gut immune responses dictate that resident innate and adaptive immune cells must coexist with the large number of microbes inhabiting the GI tract while still being able to mount an immune response against invading pathogens. Maintenance of immune homeostasis toward commensal bacteria and their microbial gene products is essential in the prevention of chronic inflammation in the gut. Overt intestinal inflammation is a hallmark of IBD. Current therapies for the management of IBD include antibiotic regimens to prevent the outgrowth and systemic dissemination of pathogenic microorganisms, as well as corticosteroids and immunomodulators to decrease the inflammatory response in the intestines. However, these therapies are not without undesirable and harmful side effects, as antibiotics also deplete the beneficial intestinal microflora, and corticosteroids and immunomodulators act as global immune suppressors, thereby increasing the risk of infection and cancer. Thus, there is a need for identifying new therapeutic agents for the treatment of such diseases.